Common Abbreviations and Terms
The following are abbreviations and terms that will be used in portions of this application. For convenience, the abbreviations and terms are collected here for reference. Details on the particular meaning of each term may be found at the first use of the term.
CNC Computer Numerical Control
Type of machine tool whose operation is controlled by a computer. May also refer to the control itself or to the programming language used by the control.
FRF Frequency Response Function
Linear relation between an applied force and the resulting system displacement, expressed in the frequency domain.
1. Field of the Invention
This invention relates to generally to CNC (computer numerical control) programming and machining. The invention is a non-contact device for measuring the dynamics of a CNC machine and its tooling, as expressed in the Frequency Response Function. The device is simpler to use than currently available devices that take similar measurements, eliminating the need for specialized training and analysis. The device can measure the Frequency Response Function both for a stationary and a rotating tool.
2. Background
CNC programming consists of generating computer commands that are passed to a machine tool that has a CNC control. The commands instruct the control on what tool paths the machine tool should take and sets various machining conditions such as the feed, or speed the tool cuts into the part, and spindle speed, or the speed with which the tool rotates when cutting the part. There are many factors that can influence whether the as-machined part meets specifications. These include incorrectly programmed tool paths (e.g. tool gouging into a design surface), tool wear causing the actual cutting surface to be off-set from the expected cutting surface, and too aggressive feed values causing—for example—the tool to break or chip. These and similar factors may be classified as “static” errors. Another class of machining errors is related to “dynamic” or vibrational effects. A key determinant of these vibrational effects is the Frequency Response Function which is a measure of the CNC machine dynamics.
The Frequency Response Function is integral to determining the effect of machine dynamics on dynamic machine errors and part quality as explained by J. Tlusty in Manufacturing Processed and Equipment, Prentice Hall, Upper River Saddle, N.J. (2000); the entire contents and disclosures of which are hereby incorporated by reference.
Part errors arising from machine dynamics may be surface location errors from forced vibrations or chatter marks due to an instability in the cutting process. The invention is specific to the determination of the Frequency Response Function and these two sources of part errors are offered only as important examples of the use of the Frequency Response Function in CNC machining.
Surface location errors occur when the machined surface is not at the location expected from the CNC part program due to cutter dynamics (T. Schmitz and J. Ziegert, “Examination of Surface Location Error Due to Phasing of Cutter Vibrations,” Precision Engineering, vol. 28, 51–62 (1999), the entire contents of which are incorporated hereby by reference). The Frequency Response Function is central to a reliable prediction of this effect, which can be as large or larger than geometric and thermal part errors.
There is a well established literature on chatter and its linkage to the dynamics of the CNC machine and its tooling as described in Y. Altintas, Manufacturing Automation (Cambridge University Press, 2000), the entire contents of which are hereby incorporated by reference. This dynamic information can be used to predict the safe depth of cut. The safe depth of cut depends strongly on the spindle speed as shown schematically in FIG. 1. A safe manufacturing operation could maintain depths of cut below D—Limit (as shown FIG. 1) or the operator can achieve much larger depths of cut and higher productivity, by operating at or near certain spindle speeds (e.g. S1 as shown in FIG. 1). Altintas summarizes decades of chatter research that demonstrates that both the limiting depth of cut (D-Limit) and the special spindle speeds (e.g. S1) depend on the dynamics of the CNC machine and its tooling.
Chatter may be attributed to various mechanisms. The most common mechanism is regenerative chatter, as described by Altintas and by Tlusty. ecently, Davies and collaborators (Davies, M. A., Dutterer, B., Pratt, J. and Schaut, A. J., On the Dynamics of High-Speed Milling with Long, Slender End Mills, Annals CIRP 47 (1), 71–76 (1998), the entire contents of which are incorporated hereby by reference) have proposed an alternate mechanism based on impact dynamics. The Frequency Response Function is central to prediction of all chatter mechanisms.
The present invention is a novel way to measure the dynamics, specifically the Frequency Response Function, of the CNC machine and its tooling. The invention allows these measurements to be made using a simple and inexpensive non-contact device. The invention allows these measurements to be made by a person skilled only in standard CNC operation, without any additional special training or complex analysis. The device may be used to measure the Frequency Response Function for a rotating, as well as a stationary, tool.
The present invention will provide CNC programmers and CNC machine operators with the dynamics information required to predict, in advance of cutting, when excessive forced vibrations and/or chatter may occur. In addition, the device will assist CNC programmers and operators in adjusting the parameters in the CNC program, such as the speed at which the tool rotates, to achieve optimal cutting conditions.
The device may be used to find the Frequency Response Function of individual tools or the measurements on one tool may be used to obtain the Frequency Response Functions for a collection of tools using a novel method and measurement strategy developed by Esterling, U.S. Provisional Patent Application No. 60/456,947, filed Mar. 25, 2003, the entire contents and disclosures of which are hereby incorporated by reference. Use of the Esterling method substantially reduces the number of required measurements with the device.
3. Description of Prior Art
The most commonly used procedure to determine the Frequency Response Function of a tool sited in a CNC is the hammer impact method. This method is described by Altintas and by N. Maia, et al., “Theoretical and Experimental Modal Analysis,” John Wiley & Sons, NY, N.Y. (1997), the entire contents of each are hereby incorporated by reference. The operator hits the tool with a calibrated hammer. The hammer impact supplies a near-impulsive force profile. The resulting displacements are detected with a displacement sensor and the combined force and displacement signals are read and analyzed to produce the Frequency Response Function. This method requires considerable experience and dexterity with the testing procedure, as the hammer impact must be as clean and reproducible as is feasible. Since the hammer impact is manual, there is considerable variation between tests and multiple impacts for a single strike are common. The analysis system associated with the hammer impact tests requires skill in the use of complex electronics and interpretation of complex charts. These tests and their interpretation could not be expected of a typical shop floor machinist.
A device similar to our invention is described in U.S. Pat. No. 6,349,600 to Davies, et al., the entire disclosure and contents of which are hereby incorporated by reference. The Davies invention seeks to provide a guide to the best spindle speeds for a CNC machinist or programmer to use in order to avoid chatter. Normally, In order to predict chatter conditions, it is necessary to know the dynamics of the machine tool/workpiece, specifically as embodied in its Frequency Response Function. This function relates the time-varying displacement of the entity in response to a time-varying force. The invention of Davies, et. al., Device for Stable Speed Determination in Machining, explicitly states—in contrast to the current invention—that the device will not measure the Frequency Response Function. The Davies device only seeks to find special spindle speeds which correspond to specially stable (chatter-free) conditions. Doing so only requires finding the natural frequencies of the system. These correspond to the frequencies that exhibit peaks in the full Frequency Response Function. This simplifies somewhat their invention requirements but, as stated in the Description Section regarding the Davies Patent, prevents them from predicting the actual chatter-free depth of cut.
The best cutting speeds determined with the Davies device may be, and often are, exceptionally high speeds and are not attainable for many CNC machines. In addition, since the Davies device provides no guidance on the stable depths of cut at these or any other spindle speeds, the operator must determine the stable, chatter-free depths of cut by trial and error.
Our invention will determine the full Frequency Response Function. The frequencies corresponding to the peaks in the Frequency Response Function will determine the natural frequencies of the system and, from these, best cutting speeds. By measuring the full Frequency Response Function and by the operative chatter theories or their extensions, the device will predict stable depths of cut both at the best speeds and over a range of spindle speeds.
Our invention will obtain more useful information than the Davies, et. al. invention and will do so in a simpler fashion. A more detailed comparison between the Davies device and the present invention is provided in the Detailed Description of the invention.